The long term goal of this project is the study of the hemodynamics of the coronary microcirculation. The emphasis on the influence of myocardial compression on small vessels requires that these studies be made on hearts with minimal mechanical restraints or interference with cardiac contraction. The approach which was developed in this laboratory to study the coronary microcirculation was a free motion technique in which the heart is visualized for a brief period of time but at the same time point during each consecutive cardiac cycle which produced the illusion of a stationary heart as viewed through the microscope. One goal of this project is to utilize this free motion approach to measure particle velocity and flow in the small epicardial vessels of the cat heart. Again the heart is illuminated at the same point in the cardiac cycle, except three flashes of light in rapid succession are provided to illuminate fluorescent latex beads at three different positions as they travel through the coronary circulation. The distance traveled is related to velocity, while the average number of beads in a length of vessel is related to the volume of the segment and thus also related to the cross sectional area of the vessel. Flows can be calculated from cross sectional area and average velocities. Particles in a vessel will travel at different velocities depending on their proximity to the vessel wall, so that knowing the distribution of particle velocities allows the determination of the flow profile within the vessel. Hemodynamic characteristics which are unique to the heart include a phasic discrepancy between arterial inflow and coronary sinus outflow, a high small vein phasic pressure, a high zero flow pressure, and major changes in vessel diameter related to myocardial contraction. Only recently have the techniques been developed to study the small vessels of the coronary system and therefore, in the past, only indirectly derived information was available. The ability to study and understand these vessel has major importance in such areas as intermittent coronary sinus occlusion, no reflow phenomenon, coronary steal phenomenon, and collateral augmentation of ischemic flow.